When membrane separation by a permeation membrane such as a RO membrane is carried out for water to be treated containing suspended solid and organic matter together with contaminant substances such as microorganism, the permeation membrane is contaminated by these substances wherein clogging of membrane by slime adhesion occurs to decrease flux and separation rate. In order to prevent the contamination of the membrane and to increase separation efficiency, there is adopted a slime adhesion prevention method wherein an oxidizing agent based on chlorine is added to feed water to be supplied to a membrane separation apparatus.
Patent document 1 (JP2006-263510A) teaches slime prevention treatment without degradation of the membrane and without formation of trihalomethane wherein a stable combined chlorine agent comprising chlorosulfamate formed from an oxidizing agent based on chlorine and a sulfamic compound is added to feed water to be supplied to a membrane separation apparatus. The sulfamic compound is exemplified by sulfamic acid or a salt thereof. The sulfamic acid is amidosulfuric acid represented by Formula [1] bellow wherein R1, R2 denotes each H or hydrocarbon group having carbon number of 1-6 independently. The oxidizing agent based on chlorine is exemplified by chlorine gas, chlorine dioxide, hypochlorous acid and salt thereof.R1R2NSO3H  [1]
While patent document 1 suggests both continuous and intermittent additions of a sterilizing agent, continuous addition is adopted in Examples. Such continuous addition requires a large amount of chemicals causing high treatment costs. Further, even in case of continuous addition, low preventing effect of slime adhesion is obtained for the water having a large number of viable cell counts and having water quality of a harsh condition due to a high slime adhesion potential.
Patent document 2 (JP2010-201312A) teaches a method of membrane separation by adding a combined chlorine agent comprising a sulfamic compound to feed water supplied to a membrane separation apparatus wherein the additive amount of the combined chlorine agent is periodically or irregularly increased to 2-10 times of the ordinary amount for a period (T) and wherein the Z value represented by the following Formula [2] is 1.0<Z<2.0.Z=(Mo×T+Mx×Tx)/(Mo×T)  [2]
(In Formula [2], Mo: Concentration of combined chlorine agent in feed water during ordinary concentration addition, T: Feeding period, Mx: Concentration of combined chlorine agent in feed water during 2-10 times concentration addition, Tx: Feeding period under concentration Mx.)
Although, in Patent document 2, during continuous addition of a combined chlorine agent comprising a sulfamic compound, a high concentration addition is performed periodically or irregularly, a low preventing effect of slime adhesion is obtained for the water having a large number of viable cell counts and having a water quality of a harsh condition due to a high slime adhesion potential. Considering the reason, if the proposed treatment is performed in the range of the increasing rate Z of 1.0<Z<2.0, the additive concentration during the high concentration addition period seems to be so important that the addition is carried out at a high concentration as possible whereby the ordinary concentration addition period of low effect becomes longer causing an increase of slime adhesion during the low effect period. On the other hand, if the low concentration period is set as short as possible, there comes out a problem that the effect by addition at a high concentration is almost lost because such a concentration in the high concentration period cannot be heightened. In Patent document 2, since a combined chlorine agent is added continuously, shock to microbes by intermittent changes in the concentration is small and the exfoliating effect to already adhered slime is presumably small.
Patent document 3 (JP2000-42544A) teaches a method of reverse osmosis membrane separation wherein a chlorine agent is added intermittently to the water to be treated under the condition that the residual chlorine concentration at the moment of addition to the water to be treated is in the range of 0.2-10 mg/L as a concentration level not forming trihalomethane, that the adding rate I represented by the following Formula [3] is in the range of 0.01-0.95, and that the residual chlorine concentration just before reverse osmosis membrane is in the range of 0.2-1 mg/L.I=(T/1440)×N  [3]
(In Formula [3], T denotes the adding period of the chlorine agent (min), N is number of additions in 1 day.)
In Patent document 3, since a chlorine agent is added intermittently at a low concentration level not forming trihalomethane, a low preventing effect of slime adhesion is obtained for the water having a large number of viable cell counts and having a water quality of a harsh condition due to a high slime adhesion potential. If a chlorine agent is added at a high concentration in order to obtain an effect, there arises problems such as trihalomethane formation and deterioration of the membrane. Although, in Patent document 3, the chlorine concentration varies by the intermittent additions of sterilizer, the shock to microbes by intermittent changes is small and the exfoliating effect to already adhered slime is presumably small because a chlorine agent is added continuously to the water to be supplied to the membrane.
Patent document 4 (JP2003-267811A) teaches a method of exfoliating slime wherein slime is exfoliated by adding to a water system a slime exfoliating agent comprising an oxidizing agent based on chlorine and sulfamic acid and/or salt thereof. In Patent document 4, it is described that a slime control agent sterilizes or controls the multiplication of microbes by denaturation of the cell membrane, or by prohibition of the enzyme reaction of bacteria, while a slime exfoliating agent disperses agglomerates of microbes to exfoliate slimes from adhered surfaces mainly by lowering the adhesiveness of adhesive substances (usually polysaccharides) residing outside of microbes, and that the slime exfoliating agent of Patent document 4 has the ability to lower the adhesiveness of polysaccharides residing outside of microbes to reveal a slime exfoliating effect.
In Patent document 4, however, the method of exfoliating slime is applied only to the slime formed in ordinary water systems such as plant cooling water systems, paper—pulp water systems, waste treating water systems, iron—steel water systems and cutting oil water systems wherein there is no suggestion for application to the slime exfoliation of a permeation membrane such as a RO membrane. Further, in Patent document 4, slime exfoliation is performed after slime adhesion has occurred. This means that the slime exfoliation is intended to be carried out in a different process to the prevention of slime adhesion. Since, in permeation by a permeation membrane such as a RO membrane, water as a solvent permeates the membrane, microbes residing in the supplied water remains at the surfaces of the membrane to form a biofilm. If the biofilm grows into slime to adhere to the membrane surfaces, the exfoliation of the slime becomes to be difficult.
In Non-patent document 1 (Microorganism Science 2), it is described that the proliferation process of unicellular microorganisms includes 1) Lag phase wherein the cell count does not vary, 2) Acceleration phase wherein the cell count increases gradually, 3) Logarithmic phase wherein the cell count increases exponentially, 4) Retardation phase wherein the specific proliferation rate lowers, 5) Stationary phase wherein the increase and decrease of viable cell count come to a standstill, 6) Accelerating death phase wherein the viable cell count begins to decrease then, and 7) Logarithmic death phase wherein the viable cell count decreases exponentially. There is also described that, when chemical stress is given to microorganisms, the action of the chemicals becomes stronger, as the concentration of the chemicals becomes higher, in such order as 1) No effect, 2) Acceleration of proliferation, 3) Inhibition of proliferation, 4) Sterilization. There are further described that the proliferation rate decreases to zero at the inhibiting concentration for proliferation and that an increasing rate of cell number becomes negative at the sterilizing concentration whereby the cell numbers decrease or becomes extinct. It is also described that, with regard to extinction, there is a logarithm law of extinction wherein a linear relation is formed between duration and the logarithm of viable cell count.